The mTOR pathway: Role in Aging and Cancer

The mammalian target of rapamycin (mTOR) protein is a key regulator
of cell proliferation and growth, integrating the influence of
upstream signals on cell division. We offer several mTOR antibody products as well as
antibodies that recognize other members of the mTOR signaling pathway
for use in immunoblotting or immunoprecipitation. Here we present:

Several signaling pathway molecules are involved in both aging and
cancer; for example, the tumor suppressor p53 has been referenced as a
modulator of cellular senescence and organismal aging (1). Other tumor
suppressors, including p16(INK4a) (2), pTEN (3) and C/EBP (4),
similarly contribute to the balance between senescence and
immortalization. The mTOR pathway integrates multiple signals to
affect lifespan, cellular responses, and contributes to disease
processes. As such, there is keen interest in evaluating new mTOR
inhibitor agents as potential drug candidates, while known mTOR
inhibitors such as rapamycin and its derivatives are useful tools for
understanding the role of the mTOR signaling pathway in greater detail.

How does the mTOR pathway play a role in both aging and cancer?
Mammalian Target Of Rapamycin (mTOR) is a serine/threonine kinase in
the insulin receptor substrate (IRS) pathway that assembles into
either of two complexes: mTOR complex I (mTORC1) or mTORC2. mTORC1 is
activated by growth factors and nutrient levels and is inhibited by
rapamycin, an immunosuppressive drug that binds to FKBP12. FKBP12 is
then in turn able to bind to the FRB domain of mTOR, thereby
inhibiting mTOR activity. mTORC1 activity is also inhibited by cell
stress, caloric restriction, and endogenous inhibitors such as FKBP38.

The Regulatory Associated Protein Of mTORC1 (Raptor) is a subunit of
the mTORC1 complex and regulates its activity. Raptor also associates
with Eukaryotic translation Initiation Factor 4E-Binding Protein 1
(EIF4BP1) and ribosomal protein S6 kinase (S6K1), promoting their
phosphorylation and upregulation of protein synthesis via mTORC1.

Rapamycin-Insensitive Companion Of mTOR (Rictor) is a subunit and
regulator of mTORC2. It may compete with Raptor for binding to mTOR.
The mTORC2 complex is activated by growth factors but, unlike mTORC1,
is insensitive to nutrient levels.

Advances in understanding how mTOR phosphorylation affects
association of mTOR with mTORC1 or mTORC2 has helped unravel the
mechanisms of action and regulatory roles of this kinase. Watanabe and
colleagues have reported four phosphorylation states for mTOR that
determine the formation of the specific complexes, as well differing
sensitivities of each complex to rapamycin and wortmannin (5). An
overview of the mTOR pathway is shown below.

The mTOR pathway has been found to play an important and possibly
pivotal role in aging and cancer. Wu and colleagues reported that the
mTOR pathway is a critical regulator of aging. Hypomorphic mice with a
75% reduction in mTOR expression relative to wild-type mice lived up
to 20% longer (6). The mutant mice were also reported to have better
balance and coordination, muscle strength and spatial learning, and
memory than the wild-type mice.

mTOR inhibition has also been correlated with reduction in other
biomarkers of aging-associated diseases such as Alzheimer's Disease.
When the natural compound arctigenin was administered to an APP/PS1
transgenic mouse model of Alzheimer's Disease, the resulting Akt/mTOR
pathway inhibition was correlated with ameliorated memory impairment
and an increase in beta amyloid protein clearance (7).

mTOR may play a role in cancer progression as well. mTOR inhibition
has been correlated with reduced recurrence of hormone receptor
positive breast cancer, possibly because mTOR regulates the PI3K/Akt
signaling network regulating cell proliferation, migration, and
survival (8). A similar suppressive effect was reported when the
PTEN/PI3K/Akt and mTOR signaling pathways were targeted as part of a
treatment strategy for hormone refractory prostate cancer (9) and
during inhibition of the mTORC1/S6K1 and PI3K/Akt pathways in
hepatocellular carcinoma (10).

Branched-chain amino acid (BCAA), often used as a treatment agent
for hepatocellular carcinoma, was found to enhance the negative
feedback loop of mTORC1/S6K1, resulting in a greater inhibition of the
PI3K/Akt pathway. BCAA treatment was also found to suppress mTORC2
phosphorylation of Akt. Immunoprecipitation experiments were performed
using the mouse monoclonal mTOR antibody (Cat.# 10343A).

Birt-Hogg-Dubé syndrome (BHDS) is an autosomal dominant
genetic disease that predisposes patients to fibrofolliculomas, lung
cysts, and renal neoplasia. BHD is a tumor suppressor that encodes
folliculin (FLCN). The mTORC1 complex was known to regulate FLCN
downstream of the mTORC1 negative regulator tuberin, the product of
tuberous sclerosis 2 gene (TSC2), but the precise mechanisms were
unknown. The authors identified multiple sites at which FLCN is
phosphorylated by an mTORC1-dependent pathway, which then affects the
interaction of FLCN with AMPK-activated protein kinase (AMPK).
Antibodies to Raptor and Rictor from Immuno-Biological Laboratories
were used in this study.

The interaction of mTORC1 with its substrates PRAS40, S6K1, Rheb,
and 4E-BP was investigated as a function of insulin and amino acid
levels. Rheb activation was found to lead directly to mTOR catalytic
activity, while PRAS40 dissociation did not lead directly to mTORC1
signaling. For immunoprecipitation experiments, anti-mTOR,
anti-PRAS40 (Pro238), and anti-raptor (R1) antibodies from
Immuno-Biological Laboratories were used.